Seismic Reflection and Electrical Resistivity Imaging Support Pre‐Quaternary Glaciation in the Rocky Mountains (Unaweep Canyon, Colorado)

Unaweep Canyon (Uncompahgre Plateau, Colorado) represents an enigmatic landscape with a complex evolution. Interpretations for its origin have ranged from ancestral fluvial erosion in the late Cenozoic to glacial erosion in the Paleozoic, or some combination thereof, with significant implications for global climatic and large‐scale tectonic reconstructions. To address the conflicting interpretations, we acquired a high‐resolution seismic reflection profile to investigate the depth, structure, and sedimentary infill in the canyon. The data set is further complemented with an electrical resistivity survey. Integrated with other geophysical and geological data, the results show an overdeepened Precambrian basement with transverse U shape and support the hypothesis of a pre‐Quaternary glacial origin. Our data constitute the first detailed image of a buried pre‐Quaternary glacial valley in North America; if substantiated with core studies, these results have far‐reaching implications for our understanding of global ice houses as well as the tectonic conditions, enabling preservation of such systems. Plain Language Summary The shape of valleys depends on the processes that created them. Broad valleys with the transverse shape of a “U” are distinct signs of glaciers, which carved the valley in the bedrock. Since old valleys are filled up with sediments, we need geophysical tools to image their structure in the buried subsurface. Using such tools, we have found a buried U‐shaped valley in western Colorado. Since we know from other observations that there were no glaciers in this specific location in the last ice age, the valley must have been created in a previous ice age. The most likely time period is the late Paleozoic, ca. 300 million years ago. Due to plate tectonics, the landmass of Colorado was located at the equator during that time. This implies that there was ice at the equator at this time, which further means there were globally much lower temperatures than we think. Our result is the first example in North America of a glacial mountain valley carved by an ancient glacier. Key Points We present the first high‐resolution seismic image of a buried paleovalley shaped by alpine glaciation in Earth's pre‐Quaternary record Our data support the hypothesis of late Paleozoic glaciation at latitudes and elevations lower than suggested by current paleoclimate models